# std::ranges::less

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Defined in header `<functional>` |  |
struct less; |  | (since C++20)

Function object for performing comparisons. Deduces the parameter types of the function call operator from the arguments (but not the return type).

### Nested types

Nested type | Definition
`is_transparent` | unspecified

### Member functions

operator() | checks if the first argument is less than the second (public member function)

## std::ranges::less::operator()

template< class T, class U > constexpr bool operator()( T&& t, U&& u ) const; |  |

Given the expression std::forward<T>(t) < std::forward<U>(u) as expr:

- If expr results in a call to built-in operator< comparing pointers, given the composite pointer type of t and u as `P`:

  - If the converted t precedes the converted u (both are of type `P`) in the implementation-defined strict total order over pointers, returns true, otherwise returns false.
  - If the conversion sequence from `T` to `P` or the conversion sequence from `U` to `P` is not equality-preserving, the behavior is undefined.

- Otherwise:

  - Returns the result of expr.
  - If std::totally_ordered_with<T, U> is not modeled, the behavior is undefined.

This overload participates in overload resolution only if std::totally_ordered_with<T, U> is satisfied.

If there exists an expression expr1 of type `T` and an expression expr2 of `U`, such that the comparison results of expr1 and expr2 violate strict total ordering (rules are defined below), the behavior is undefined.

The comparison results of expr1 and expr2 follow strict total ordering only if exactly one of the following expressions is true:

- std::ranges::less{}(expr1, expr2)
- std::ranges::less{}(expr2, expr1)
- std::ranges::equal_to{}(expr1, expr2)

### Notes

Unlike std::less, `std::ranges::less` requires all six comparison operators `<`, `<=`, `>`, `>=`, `==` and `!=` to be valid (via the `totally_ordered_with` constraint).

### Example

| This section is incomplete Reason: no example

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3530 | C++20 | syntactic checks were relaxed while comparing pointers | only semantic requirements are relaxed

### See also

less | function object implementing x < y (class template)

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